1. Field of the Invention
The present invention relates to a fundus camera having an automatic focusing function.
2. Description of the Related Art
Japanese Patent Application No. 08-150121 discusses a fundus camera in which either a visible light mode or a near-infrared light mode is selected for observation of fundus, and an automatic focusing is performed based on a fundus image for photographing the fundus image using visible light.
Conventionally, in auto-focusing by a sharpness detection system, focus evaluation values are obtained based on high frequency components within a predetermined area of a fundus image while moving a focusing lens. The curve profile (mountain shape) of changing values needs to be obtained to perform auto-focusing.
To obtain the profile and a peak of a curve, a focusing lens is controlled as follows: first, a focusing lens moves between a − endpoint and a + endpoint (in either direction) within a moving range for scanning to obtain focus evaluation values, so that a peak of the profile of the scanned focus evaluation values is detected. After the detection, when the following focus evaluation value drops below a predetermined value, the scanning is stopped. Then, the focusing lens returns to the position where the peak value of the focus evaluation values can be acquired, and photographing is performed thereat.
FIG. 8 illustrates the above control. The horizontal axis represents moving range of a focusing lens between a + end point for the limit of a + diopter and a − endpoint for the limit of a − diopter. The vertical axis (on the + side) represents focus evaluation value, each of which is, for example, a sum of values for high frequency components within a predetermined area of a fundus image.
With respect to an image, the contrast is higher and thus the focus evaluation values are higher at the positions closer to the in-focus position of an image. The curves in FIG. 8 consists of focus evaluation values for a case where a focusing lens is moved throughout the moving range for scanning. Actually, however, apart of the range is scanned, resulting in a curve only for the range.
The vertical axis (on the − side) represents time with respect to movement of a focusing lens and a sequence of scanned focus evaluation values. The solid line Mx represents a simple movement of a focusing lens, and the broken line Sx represents scanning for acquiring focus evaluation values, wherein x is the order of movement.
In FIG. 8, each control of a focusing lens starts at the position marked by a double circle, and ends at the position marked by a circle. The position marked by the triangle on the horizontal axis represents the position of a focusing lens in the previous focusing sequence. In the case of FIG. 8, the focusing lens was moved as illustrated in M1, S1, and M2 in this order for photographing.
So far, however, no fundus camera has come into use in which the controls of automatic focusing in a visible-light observation mode and a near-infrared light observation mode are optimized to reduce the time required for automatic focusing.